Patent GB-889013-A (GB)
Title: Improvements relating to character writing devices
Inventor: INGHAM WILLIAM ELLIS
IPC: G11B7/003
Filed: 1957-05-21
Abstract: 889,013. Cathode-ray tube circuits. ELECTRIC & MUSICAL INDUSTRIES Ltd. May 15, 1958 [May 21, 1957], No. 16026/57. Class 40(7). [Also in Group XL(b)] In a device for writing characters or symbols on the screen of a cathode-ray tube by the application of appropriate X and Y coordinate deflection waveforms, the waveforms are derived by scanning a record medium. In a first arrangement, Fig. 1, which shows the record for the X co-ordinate of the numeral "0", the record is in the form of discrete areas in which are recorded alternately representations of one and the other of two different signal levels, the ratio of the areas varying along the record in accordance with the desired variation of the co-ordinate waveform. By scanning along the track there is derived a pulse signal having a mark-tospace ratio which varies in accordance with the waveform and from which the actual waveform may be delivered by application of the pulse signal to a limiter and low-pass filter. With the record arranged as shown in Fig. 1, the derived electrical pulses are of substantially constant repetition frequency. In a second arrangement, Fig. 3(a), areas representing one signal level are of constant width and are separated by areas of variable width whereby the derived pulses are of variable repetition frequency but of constant duration. The recording may be optical in the form of a film with opaque and transparent areas to represent the two different signal levels, the scanning being effected by a cathode-ray tube flying-spot scanner and photo-electric cell, Fig. 2 (not shown). The record tracks for each of a number of characters are arranged horizontally, one above another, and a character selection circuit is arranged to shift the cathode-ray spot vertically on to the appropriate track before effecting a horizontal scan. A separate scanner is provided for the X and Y co-ordinate waveforms. Alternatively a common scanner in the form of a doublebeam tube may be utilised, Fig. 5 (not shown). In a third arrangement, Fig. 3(b), the record is in the form of an opaque or transparent track varying in width along its length in accordance with the desired waveform. The track is scanned photo-electrically utilising a light spot oscillating between the limits PQ and moving relative to the record as indicated by arrow 12. Fig. 4 (not shown) illustrates apparatus in which record tracks for a number of characters are arranged as concentric circles on a rotating disc. A photo-electric cell is associated with each track, the waveforms for all the characters being generated simultaneously and selected subsequently by a switching circuit. Reference is also made to the use of records carried by a drum and records which are light reflecting. The recording may be magnetic with magnetized and unmagnetized areas, the scanning being effected by a magnetic head. In this case the signal derived by scanning comprises brief impulses indicating the transitions from one area to the next. These impulses are employed as timing pulses in the generation of the pulse signal having a varying mark-to-space ratio. As an alternative the magnetic recording may be in the form of a high frequency oscillation which is modulated on and off.